1. Field of the Invention
The present invention relates to an optical backplane for carrying out signal connection between a plurality of circuit substrates (boards), which are mounted in information processing equipment such as a router, a server, or the like, by optical signals, and to a light transmission method.
2. Description of the Related Art
Recently, as a broadband network expands, an amount of information distributed through the network is increased. Consequently, it is required to increase the information processing capacity of information processing equipment such as a router, a server or the like. However, the limit of transmission speed of an electric substrate used in the equipment acts as a drawback to the increase of the capacity of the equipment. To further increase the information processing capacity of the information processing equipment by overcoming the drawback, it is effective to connect between boards accommodated in the equipment to each other by optical signals.
Ordinarily, the mutual optical connection of the interior of information equipment can be achieved by inserting a plurality of boards such as signal input/output boards, switch boards, and the like to an optical backplane, on which light transmission paths such as optical fibers and the like are laid, at right angles. Electric signals on a board is converted into optical signals by a photoelectric conversion module and sent to the optical backplane, and the optical signals transmitted on the optical backplane are returned to electric signals and transmitted to other board.
It is required to increase the information processing capacity of a piece of the equipment by increasing the density of boards by reducing the mounting intervals therebetween, as a requirement for making use of the advantage of the optical backplane.
A structure as shown in FIG. 1 is known as a conventional structure of the optical backplane. FIG. 1 is schematically drawn based on a photograph found in “Research and Development of Ultra High Density Electronic SI Technology, Development of Technology for Rationally Using Energy”, Report of Accomplishment of Study in 2000, Association of Super-Advanced Electronics Technologies, pp. 377, (2000).
A plurality of optical fibers 2 placed in a juxtaposed state (side by side) and formed into a sheet state are attached on an optical backplane 1 (in the above document, the portion noted as Optical Fiber Board), the edge portions of the plurality of juxtaposed optical fibers 2 are bent such that the optical fibers are at right angles to the optical backplane 1, the optical fibers are attached to a connector 3 (in the above document, the connector noted as Right-Angel Connector), and the optical connector 3 is optically connected to an optical connector 5 disposed to an edge portion of a board 4. In this structure, the fibers in the edge portion of an optical fiber array are disposed in parallel with a surface of the board 4.
It should be noted that the above document and Japanese Patent Application National Publication No. 2003-515785 (FIGS. 2 to 4 and 9 to 12, and paragraph Nos. (0022) to (0023), (0027)) and Japanese Patent Application National Publication 2004-507785 (FIGS. 2 to 3 and 9 to 10) disclose that the disposing direction of a plurality of optical fibers, which are disposed to and accommodated in an optical connector passed through an optical backplane, is at a right angle to a circuit substrate, as a technology relating to the present invention.
In a structure in which a fiber disposing direction at the edge of an optical fiber array is in parallel with a surface of a board as described above, optical fibers are designed and laid to bypass a connector portion on an optical backplane as shown in FIG. 1. This is because when the optical fibers are laid to overlap a portion where a connector is attached, it is difficult to attach the connector. As a result, the optical fibers must be bent twice in a vertically standing-up portion 9 at the edges of the optical fibers and in a bypass portion 8 of the connector portion. A disadvantage arises here in that the optical fibers have a lower limit in a bending radius. According to the above documents, to bend, for example, ordinary multimode fibers twice, an interval between adjacent connectors must be set to a certain degree of length, for example, to 45 mm or more, and thus the mounting interval between boards cannot be reduced to less than 45 mm. Because of the above reasons, the structure of the conventional optical backplane has a disadvantage in that the mounting density of the optical backplane cannot be increased due to necessity of bypassing the connector portion when the optical fibers are laid and to the lower limit of the fiber-bending radius.